Communication networks based on optical fibers are becoming increasingly popular because of their high data transmission capacities and the low cost of the optical fibers. In a typical communication network, a number of users communicate with one another over an optical fiber arranged in a loop by sending signals along the fiber in a predetermined direction. For example, in a telecommunications network each subscriber communicates with a central office over a fiber that is arranged in a ring with the subscriber and central office stations disposed along the ring. If the fiber is broken, communication between one or more of the users and the central office will be interrupted. In principle, these users can still communicate with the central office by sending messages along the uninterrupted portion of the loop. However, this requires that the direction of propagation along the fiber be reversed over a portion of the fiber.
Unfortunately, the fiber ring typically includes components that are unidirectional in nature such as optical amplifiers and isolators. A typical optical amplifier consists of a doped fiber between two optical isolators that prevent light generated in the doped fiber from propagating in the backward direction around the fiber. To reverse the direction of propagation in response to a fiber break, duplicate optical isolators configured to propagate signals in the opposite direction are included in the network. These components are inserted into the fiber in place of the corresponding components by utilizing bypass switches. Such bypass arrangements substantially increase the cost and complexity of the optical network, and hence, it would be advantageous to avoid these bypass arrangements.
Broadly, it is the object of the present invention to provide an improved optical amplifier.
It is another object of the present invention to provide an optical isolator whose direction of light transmission can be reversed by applying a control signal to the isolator without the need to utilize bypass switches and additional isolators.
It is yet another object of the present invention to provide an optical amplifier having a switchable direction of propagation for the amplified light signal.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.